1. Field of the Invention
The invention relates to a cooling system of an engine (internal combustion engine) and, more particularly, to a coolant temperature sensor abnormality determination apparatus and a coolant temperature sensor abnormality determination method that determine the presence or absence of abnormality of a coolant temperature sensor that detects the temperature of a coolant of the engine.
2. Description of Related Art
With regard to an engine mounted in a vehicle or the like, a coolant jacket as a coolant passageway is provided in the engine (a cylinder block or a cylinder head), and the entire engine is cooled (or warmed) by circulating a coolant via the coolant jacket by a coolant pump. In conjunction with such a cooling system, there exists a technology in which a coolant outlet of the engine is provided with a changeover valve, and while the engine is cold, the changeover valve is closed to stop passage of the coolant within the engine (within the coolant jacket) (to perform an in-engine coolant stop) so that quick warm-up of the engine is accomplished (e.g., see Japanese Patent Application Publication No. 2009-150266 (JP-A-2009-150266)).
Besides, the cooling system of the engine is provided with a coolant temperature sensor that detects the temperature of the coolant. As a technology of detecting abnormality of the coolant temperature sensor, there exists a coolant temperature sensor abnormality detection method described in Japanese Patent Application Publication No. 2007-192045 (JP-A-2007-192045). In the method described in the publication JP-A-2007-192045, if a temperature difference between the temperature detected by the coolant temperature sensor that detects the coolant temperature of the engine and the temperature detected by an intake air temperature sensor that detects the temperature of intake air of the engine is outside a predetermined range, it is determined that there is “abnormality of the coolant temperature sensor” or “a block heater attached or the high-temperature soaking”, and, after that, if the coolant temperature value detected by the coolant temperature sensor declines when the coolant pump is driven, it is determined that abnormality of the coolant temperature sensor is not present but an attached block heater or the high-temperature soaking is present.
By the way, in the cooling system that performs the aforementioned in-engine coolant stop, if the foregoing abnormality detection method is applied to the determination regarding abnormality of the coolant temperature sensor, it sometimes happens that although the coolant temperature sensor is normal, the coolant temperature sensor is determined as being abnormal (false abnormality determination). That is, in the foregoing cooling system, when the engine has just been started (while the engine is cold), passage of the coolant within the engine is stopped by closing the changeover valve provided at the coolant outlet of the engine. During this state, even if the coolant pump is driven, the coolant does not flow into the engine (into the coolant jacket) from outside (the actual coolant temperature does not change); therefore, since the coolant temperature detected by the coolant temperature sensor does not decline, it is sometimes falsely determined that the coolant temperature sensor is abnormal although the coolant temperature sensor is normal.